TIN OXIDE BASED GAS SENSORS AND METHOD FOR THEIR MANUFACTURE

Abstract: A method for fabricating gas sensors having reproducible gas sensitivity characteristics and the gas sensors so produced. The method comprises the steps: a) hydrolysis of sterically hindered tin(IV) alkoxide dissolved in a polar organic solvent in the presence of a polymer of relatively low molecular weight having a functional group capable of coordinating to tin; b) treatment of the resulting solution with a strong mineral acid to remove the polymer; c) isolation of the solid suspension; d) deposition and subsequent heat treatment at temperatures below 500 DEG C of an amount of the solid on a suitable substrate to produce a gas sensor head and e) sensitization of the gas sensor head.

Description: Tin Oxide Based Gas Sensors and Method for their Manufacture This invention relates to improvements in tin oxide based gas sensors and in particular to improvements in the method of fabricating these sensors. Throughout this document tin oxide is used to denote tin (IV) oxide ie Sn0 2 unless the context of its usage clearly shows otherwise. The use of tin oxide in the fabrication of gas sensors is well known. Typical sensor fabrication methods, for example those described in the patents UK 1 282 993 and UK 1 288809 and in the paper by N. Butta et al in Sensors and Actuators B,2 (1990) pgs 151-lβl, involve the sintering of chemically doped tin oxide powder at temperatures between 500 to 1000 °C to form porous pellets. Inherent in the use of the high temperatures to produce these pellets is the lack of control of their structure and morphology which leads to irreproducible behaviour between sensors. Additionally, commercially available sensors produced using this method do not possess the sensitivity to allow the detection of flammable gases in concentrations at the sub parts per million (ppm) level. It is the aim of the present invention to provide a tin oxide based gas sensor capable of detecting flammable gases at the sub-ppm level by using a method of fabrication which employs temperatures below 500°C. Tin oxide is an ionic solid which generally behaves as an n-type semiconductor due to slight non-stoichiometry arising from the variable oxidation state of tin. It is generally agreed that the function of tin oxide as a gas sensor depends on the initial sensitization of the sensor through adsorption of oxygen onto the surface of the tin oxide which removes conduction band electrons from the n-type material and so reduces the conductivity. When a flammable gas reaches the oxygenated tin oxide surface it removes the surface oxide species by chemical oxidation reactions, the product gases are desorbed from the surface and the electrons are returned to the conduction band. This causes an increase in conductivity of the tin oxide which, in a fabricated gas sensor is detected either by a change in the voltage across or the current through the tin oxide. It can be appreciated from the above that in order to maximise the sensitivity of a tin oxide based sensor it is desirable to produce tin oxide with a high surface area (so that the surface reactions can dominate any bulk processes) and with a low initial conductivity (so as to minimise the number of charge carriers required to produce a detectable change) . The use of high temperatures in conventional fabrication methods tends to reduce the surface area of the tin oxide and also tends to increase the n-type conductivity by increasing the non-stoichiometry. Additionally, the use of such high temperatures in the presence of impurities (often present in the materials used in the manufacture of tin oxide) can cause uncontrollable changes to occur in the bulk defect chemistry of the tin oxide which further hinders the fabrication of sensors having reproducible behaviour. It has now been found that by producing tin oxide using a Sol-Gel process a sufficiently coherent, substantially stoichiometric material can be obtained which can be formed into gas sensor heads without recourse to the use of excessive temperatures. As a result the tin oxide thus produced retains a high surface area and also has a low initial conductivity and is therefore well suited to the production of highly sensitive flammable gas sensors. According to a first aspect of the present invention there is provided a method of fabricating tin oxide gas sensor heads comprising the following steps: a) hydrolysis of sterically hindered tin(IV) alkoxide dissolved in a polar organic solvent in the presence of a polymer of relatively low molecular weight having, a functional group capable of coordinating to tin; b) treatment of the resulting solution with a strong mineral acid to remove the polymer; c) isolation of the solid suspension; d) deposition and subsequent heat treatment at temperatures below 500°C of an amount of the solid on a suitable substrate to produce a gas sensor head and e) sensitization of the gas sensor head. If the hydrolysis of the tin alkoxide is too rapid then a poorly linked network results. Since the rate of hydrolysis is dependent on the degree of polarisation of the tin alkoxide, with hydrolysis occurring more rapidly for more highly polarised alkoxides, it is preferable to use a tin alkoxide having a largely covalent character, for example tin-t-butoxide or tin-isopropoxide, in order to reduce the hydrolysis rate. An advantage of carrying out this hydrolysis in the presence of the polymer is that because of the coordinating effect the tin oxide nucleates around the polymer chain. The subsequent removal of this polymer leaves a 'hollow' shell of tin oxide which ensures high surface area and good porosity. Most usefully, the rate at which the solid condenses during the hydrolysis step can be further controlled by carrying out the hydrolysis in the presence of a chelating agent, such as acetylacetone, having ligands which can coordinate to the tin after hydrolysis to hinder the formation of the oxide structure, thereby increasing control of the condensation. In order to help control the structure and morphology of the solid and hence sensor behaviour it is essential that the heat treatment is carried out at temperatures below about 500°C. The temperature employed should be as low as is practicable for control of the physical properties of the solid whilst being sufficiently high to ensure sufficient cohesion between the solid particles without causing sintering of these particles and therefore the temperature employed for heat treatment should advantageously be substantially in the range of *400°C to 500°C. According to a second aspect of the current invention there is provided a tin oxide based gas sensor head fabricated according to the method above described. An example of the use of this method of fabricating tin oxide based gas sensors will now be described in greater detail. High purity (nominally 99-992 pure) tin-t-butoxide (0.375g), supplied by INORGTECH Specialist Chemicals of Mildenhall United Kingdom, is dissolved in Tetrahydrofuran (10.6ml) in a Schlenk flask. Polyacrylic acid (0.066g) , having a molecular weight of approximately 2000, is then added and the resulting solution stirred under nitrogen gas for 18 hours. Water (1ml, about 10 molar equivalents per remaining alkoxide ligand) is added to hydrolyse the resulting solution which is stirred for another 3 hours. Concentrated nitric acid (0.5ml) is then added to remove the polymer and this final solution is filtered and washed with water and acetone to remove the nitric acid and any nitrates formed by the acid. The isolated solid is then oven-dried at ll8°C for 30 hours. An amount of the dried solid is then mixed with a solvent, for example THF, to form a slurry which is deposited on a standard Rosemount alumina substrate fitted with gold interdigitated electrodes and a platinum heater on the obverse side, to form the gas sensor head. By varying the current through the platinum heater the sensor head can be used at different operating temperatures which provides the opportunity for selective chemical sensing. This selectivity arises from the fact that the reactions of different flammable gases on the sensor surface have different activation energies and so their rates of reaction may be controlled by varying the supply of thermal energy to the sensor. Once the slurry is dried the head is heated to -400°C for 48 hours in dry air. This generates a minimal amount of cohesion between the solid particles without greatly reducing the tin oxide surface area through sintering and also sensitizes the device. The tin oxide of the gas sensor head is then further sensitized by heating at 26θ°C for at least 10 two minute periods alternatively in 1% methane in air and then in clean air. The typical performance of gas sensors comprising tin oxide based gas sensors head fabricated according to the method described in this example is discussed below in relation to the sensing of toluene as an example only and with reference to the accompanying drawings in which: Figure 1 is graphical comparison of the performance of a sensor according to the present invention against that of a commercially available sensor. Figure 2 shows the response of a sensor according to the present invention to concentrations of up to 100 ppb toluene in air. Figure 3 is a calibration curve of sensor output against toluene concentration. Figure 4 shows the variation in sensitivity (defined as 100 * [conductance in gas - conductance in air] / [conductance in air] ) of two sensors to acetone with temperature. Figure 5 shows the response of a sensor according to the present invention to concentrations of up to approximately 36 ppm xylene. The performance of a commercially available metal oxide gas sensor, supplied by Envin Scientific Instruments of Ayleburton Gloucester, UK (hereinafter referred to as the TGS sensor) , is compared with that of a sensor according to the present invention (hereinafter referred to as the SGS sensor) in detecting 15.6 ppm toluene vapour in air. Both sensors were exposed cyclically to toluene in air then to clean air and their output signals monitored as a function of time. It is obvious from the foregoing discussion that when the sensor is exposed to the toluene vapour in air the conductivity of semiconductor is expected to increase thereby increasing the sensor output current. Conversely, when the sensor is exposed to clean air again the conductivity of the semiconductor should decrease producing a corresponding decrease in the sensor output current. Referring now to Figure 1, the upper trace A is representative of the output signal, in micro amps (μA) , provided by the TGS sensor when operated under optimum conditions and the lower trace B is representative of the output signal provided by the SGS sensor, again operating under optimum conditions. / - The lower trace B shows that the SGS sensor provides a clearly measurable increase in the conductivity of the tin oxide, manifested as an increase in the output signal, when exposed to the toluene vapour which is reversed when exposed to clean air. This behaviour is seen to be reproducible over the five cycles depicted. In contrast, the upper trace A shows that the TGS sensor provides little change in the output signal over the five cycles and so could not be reliably used to detect toluene vapour at this low level. The time response of this SGS sensor to pulses of up to 100 parts per billion (ppb) of toluene vapour in air is shown in Figure 2 in which pulse A corresponds to exposure to 60 ppb toluene vapour, pulse B to 70 ppb, pulse C to 80 ppb, pulse D to 90 ppb and pulse E to 100 ppb. It can be seen that the size of output signal, measured in nano amps (nA) , is dependent on the concentration of toluene present and that the signal to noise ratio suggests an output current as low as O.lnA should be detectable. The dependence of the output signal of the SGS sensor on toluene concentration is shown over a wider concentration range in Figure 3- The extrapolated line suggests that for a sensor current of O.lnA (log value of -1) the detection limit for toluene is of the order of 1 ppb. Temperature profiles of sensitivity of two different SGS senors, denoted by the symbols ■ • respectively, are shown in Figure 4. As can be seen these two sensors produce substantially similar temperature profiles to 16.4 ppm acetone in air and serves to illustrate that the performance of these different detectors are substantially similar and highlights an advantage of using the method of fabrication according to the present invention. The narowness of these profiles also provides an indication of the chemical uniformity of the sensor surfaces prepared according to the method of the present invention as it indicates the limited range of catalytic sites at which the gas can react and hence the limited range of activation energies and associated useful thermal energies. Sensors fabricated according to the present invention are also useful in detecting other gases, for example the time response of an SGS sensor to pulses of up to approximately 36 parts per million (ppm) of xylene vapour in air is shown in Figure 5» Here pulse A corresponds to exposure to 9- PPm xylene vapour, pulse B to 11.2 ppm, pulse C to 14.5 ppm, pulse D to 20.5 ppm and pulse E to 35-2 ppm. It can be seen that the size of output signal, measured in micro amps (μA) , is dependent on the concentration of xylene present.

Claims: - 9 Claims 1. A method of fabricating tin oxide gas sensor heads comprising the steps: a) hydrolysis of sterically hindered tin(IV) alkoxide dissolved in a polar organic solvent in the presence of a polymer of relatively low molecular weight having a functional group capable of coordinating to tin; b) treatment of the resulting solution with a strong mineral acid to remove the polymer; c) isolation of the solid suspension; d) deposition and subsequent heat treatment at temperatures below 500°C of an amount of the solid on a suitable substrate to produce a gas sensor head and e) sensitization of the gas sensor head. 2. A method as claimed in Claim 1 characterised in that the sterically hindered tin alkoxide has a largely covalent character. 3. A method as claimed in Claim 2 characterised in that the sterically hindered tin alkoxide is tin-t-butoxide. 4. A method as claimed in Claim 2 characterised in that the sterically hindered tin alkoxide is tin-isopropoxide. 5. A method as claimed in any of the preceding claims characterised in that polymer in the presence of which the hydrolysis is carried out is polyacrylic acid. 6. A method as claimed in any of the preceding claims characterised in that strong mineral acid is nitric acid. 7. A method as claimed in any of the preceding claims characterised in that the heat treatment is carried out at temperatures substantially in the range of between 400°C and 500°C. 8. A method as claimed in claim 7 characterised in that the heat treatment is carried out at substantially 400°C. 9. A method as claimed in any of the preceding claims characterised in that the sensitization of the gas sensor head comprises the step of heating the head to 260°C for at least 10 two minute periods alternatively in 1% methane in air and then in clean air. 10. A method as claimed in any of the preceding claims characterised in that the hydrolysis is carried out in the presence of a agent having ligands which can coordinate to the tin after the hydrolysis to hinder the formation of the oxide structure. 11. A gas sensor comprising a tin oxide gas sensor head fabricated according to a method as claimed in any one of the preceding claims.

Inventors: WILSON ALAN, WRIGHT JOHN DALTON, WILSON, ALAN, WRIGHT, JOHN, DALTON
Applicants: SECR DEFENCE, WILSON ALAN, WRIGHT JOHN DALTON, THE SECRETARY OF STATE FOR DEFENCE, WILSON, ALAN, WRIGHT, JOHN, DALTON
Application year: 1994
Main classification: G01N27
IPC codes: G01N27